B-Lymphocyte cell lines were established using Epstein-Barr virus from peripheral blood of normal volunteers, juvenile-onset diabetics, and patients with extreme insulin resistance. Cells from patients with juvenile-onset diabetes, lipoatrophic diabetes, and Type B extreme insulin resistance had normal l25I-insulin binding. A patient with Type A extreme insulin resistance and a patient with leprechaunism had decreased binding of l25I-insulin. This resulted from a decreased number of insulin receptors. The residual insulin receptors appeared to have normal properties.